Method for disposal of refuse by degasifying and gasification thereof

ABSTRACT

A method for disposal of refuse by degasifying and gasification thereof in a retort having gas exhaustion near its outlet end, and in which the refuse is heated to such a degree that within the retort a gasification zone is maintained with a temperature of not below 900*C, by which new method a substantial part of the heat required for heating the refuse within the retort is obtained by means of steam superheated to a temperature of above 900*C, preferably above 1200*C, which steam is introduced into the retort. Preferably the steam or a part thereof is conducted through the retort co-current with the refuse, but most conveniently a part of the superheated steam is introduced into the retort between the refuse inlet and the gasification zone. Also it is recommended that the refuse is preheated to a temperature of 500* to 700*C before being influenced by the superheated steam. In many cases it may be appropriate that also at least a part of the retort wall is directly heated. Most advantageously a retort of the type corresponding to a rotary kiln is used.

United States Borggreen et al.

atet 1 METHOD FOR DISPOSAL OF REFUSE BY DEGASIFYING AND GASIFICATIONPrimary Examiner-Joseph Scovronek THEREOF Attorney-Spencer & Kaye [75]Inventors: Georg Borggreen, Kolding; Jorn Kragh, Hillerod; Erik Holm[57] ABSTRACT Laustsen Koldmg an of Denmark A method for disposal ofrefuse by degasifying and [73] Assignee: Georg Borggreen, Kolding,gasification thereof in a retort having gas exhaustion Denmark near itsoutlet end, and in which the refuse is heated to such a degree thatwithin the retort a gasification [22] Flled' 1971 zone is maintainedwith a temperature of not below [211 App]. No.1 117,829 900C, by whichnew method a substantial part of the heat required for heating therefuse within the retort is obtained by means of steam superheated to atempera- [30] Forelgn Apphcanon Pnonty Data ture of above 900C,preferably above 1200C, which Feb. 24, 1970 Denmark 884/70 steam isintroduced into the retort Preferably the steam or a part thereof isconducted through the re- U.S. Cl. t l l t t co current th f b t most ily [51] 'f Cl Clo-l 3/06 C101 3/20 F23g 5/00 a part of the superheatedsteam is introduced into the [58'] Fleld of Search 48/209, 111, 197 A,retort between the refuse inlet and the gasification 48/197 197 64 99851, zone. Also it is recommended that the refuse is pre- 201/21, heatedto a temperature of 500 to 700C before being influenced by thesuperheated steam. In many cases it [56] References C'ted may beappropriate that also at least a part of the re- UNITED STATES PATENTStort wall is directly heated. Most advantageously a re- 3,471,27510/1969 Borggreen 48/209 tort 0f the yp Corresponding to a rotary kilnis used- Rl5,320 3/1922 McDonald 48/209 1,590,279 6/1926 Broome 48/85.1x 12 Chums, 1 Drawlng F'gure l6 7 GAS EXHAUST HEATING 4 I3 I MEDIUM l9STEAM i i i SUPERHEATED STEAM\SOURCE PATENTEBnuv 131915 3; 771, 263

7 V GAS EXHAUST HEATING ;J|2 MEDIUM |9 STEAM SOURCE FIREPLACESUPERHEATED STEAM SOURCE METHOD FOR DISPOSAL OF REFUSE BY DEGASIFYINGAND GASIFICATION THEREOF This invention relates to a method for disposalof refuse, including for instance rubbish, sewage sludge and low-valueor waste fuels by degasifying and gasification thereof in a retort, inwhich the refuse is heated to such a degree that within a gasificationzone a temperature .of not below 900C is maintained.

Disposal of refuse by degasifying and degasification has many advantagesover other methods of refuse destruction, such as for instance overincineration of refuse in incinerators, thus especially a far smallerair contamination, as this is limited to an increase of the CO -contentof the air, in any case if part of the gas produced is used forgeneration of the heat required for the destruction of the refuse in theretort. If, furthermore, the destruction in a retort is carried outthrough high-temperature gasification, i.e. in such a way that in theretort zone having maximum temperature a gasification temperature occursresulting in an acceptable reaction velocity, i.e. a temperature of notbelow 900 centigrade, further those hardly useful substances, such assoot and tarry substances, which unavoidably result by low-temperaturegasification or pyrolysis of the refuse, i.e. through gasification ofthis in an atmosphere free from or deficient in oxygen at temperaturesup to 820 centrigrade, will be avoided, just as it also will be possibleto obtain gasification of the major part to the carbon contained in therefuse.

Hitherto for disposal of refuse by high-temperature gasificationthereof, retorts have been used substantially of the same types as usedfor gasification of coal, i.e. retorts having an oblong, narrowcross-section. Per

se such retorts are well suited for the destruction of refuse, but have,owing to the low specific weight of the refuse, a low capacity resultingin relatively high construction and working expenses.

It has been tried instead of retorts of the said types to use retortshaving substantially larger sectional dimensions, such as is actuallyknown from lowtemperature gasification plants, and which should increasethe capacity of each retort rather substantially, but such retorts haveproved to be incapable for obtaining or maintaining the requisite hightemperatures at the centre of the retort, as the refuse itself as wellas the products occurring during the gasification are highlyheat-insulating, one of the reasons being their contents of a highpercentage of gas-filled cavities.

An object of the present invention is to provide a method as mentionedabove, which renders it possible to use retorts of large sectionaldimensions and accordingly having a large capacity, and still to obtainthe requisite high temperature across the whole of the crosssection ofthe part of the retort, where the temperature is to be highest, and sothat the total calorific value of the gas gained by each weight unit ofrefuse in any case is not smaller than that obtained in connection withthe retorts hitherto used.

According to the present invention this is achieved thereby that aretort having gas exhaustion between the gasification zone and the ashoutlet of the retort is used, and that a substantial quantity of theheat required for the destruction of the refuse in the retort is fed tothe refuse in the form of steam superheated to a temperature exceeding900C, preferably exceeding 1200C. Hereby is primarily obtained that therefuse is heated directly by the superheated steam, which isapproximately evenly distributed in the retort so that within eachcross-section of the retort the refuse is practically uniformly heated.Furthermore is obtained that the products formed at the beginning of theheating of the refuse through degasification thereof, and of which someare fats, as a result of the gas-exhaustion taking place after thegasification zone, are sucked through the latter and here affected bythe high temperature, which together with the large excess of vapour,also serving as a gasification means, will have the effect of thesesubstances as well as primarily formed carbon, such as for instance morecomplex compounds originating from plastics in the refuse, react withthe vapour, substantially forming C0, C0 H and CH For this reason theuse of the new method results in substantially more gas being producedfrom each weight unit refuse than obtainable ,by the indirect heating ofthe refuse hitherto used, and, at the same time, the amount of slag andashes will be substantially reduced, and the slag and ashes will bepractically free from carbon. While thus hitherto slag and ashes fromrefuse gasification plants have contained up to 40 percentage by weightof carbon, ashes and slag formed in connection with refuse destructionaccording to the invention will contain only a very small weightpercentage of carbon. Certainly, the gas produced will have a very largecontent of vapour, but the major part thereof may be removed bycondensation, through which very large quantities of heat may beregained, but even after the condensation of the vapour the quantity ofgas will be substantially larger relative to the amount of refuse thanhitherto achieved. It is true that the gas will be of a type having asomewhat smaller calorific value per normal cubic metre than gas fromretorts heated from outside, but as a consequence of the large quantityproduced, the total calorific value will be essentially higher, oftenmore than twice the total calorific value of the gas obtained from thesame refuse quantity in the retorts hitherto used. The gas is forinstance excellently suitable for heating of the boiler, in which thesuperheated steam is'produced. However, even if all the heat requiredfor degasifying and gasification of the refuse is produced by the gasformed, there will still be a surplus of gas, which may be used by adistrict heating or power station attached to the refuse destructionplant. On the other hand, the gas obtained is less suitable for directuse as town gas, but may be rendered useful as such through carburettingorby being used as a thinner for gas produced in another way and havinga high calorific value per normal cubic metre.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic diagram ofan embodiment of a refuse disposal system for carrying out a disposalmethod according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The drawing provides a schematicillustration of a system for disposing of refuse. The refuse is fed froma source 17 to a preheater 18 where the refuse is heated to atemperature of between 500 and 700C. The refuse is fed from thepreheater 18 into a rotary retort 11 in which the refuse is heated to asufficient temperature for degasification and gasification to occur. Therefuse in the retort 11 is primarily heated by superheated steam whichis supplied to the retort by superheated steam source 14. This source 14supplies the superheated steam to the retort 11 in two locations, thefirst being in the area of the inlet of the retort and the second beingin a position between the gasiflcation zone and the inlet.

A steam source 19, located in a heating medium 12, supplies steam to thesuperheated steam source 14. The heating medium 12 is also connected tothe superheated steam source 14 for providing a source of heat to thelatter. A fireplace 13 supplies heat to the heating medium 12. Theheating medium 12 additionally supplies heat to the walls of the retortso as to help maintain uniform heating within the retort.

After the refuse has been treated within the retort, the ashes which areproduced are collected within the ash outlet 15 and the gases which areproduced within the gas exhaust 16. A portion of the gases passingthrough the exhaust 16 can be recycled back into the fireplace forfurther burning.

It has proved to be most advantageous if the superheated steam passesthrough the retort co-current with the refuse, so that the refuse isheated intensely immediately it has come into the retort, whereby thedegasification period will be relatively short, and the gasificationperiod correspondingly longer, which is an advantage when thedecomposition is to be as complete as possible.

However, the introduction of the superheated steam in the said mannermay have the effect that too large a quantity of the heat contained inthe steam is used for the degasification, and if too large a consumptionof steam is to be avoided, therefore, for ensuring the requisite hightemperature in the gasification zone it may at times be expedient thatthe superheated steam or a part thereof is fed into the retort betweenthe gasification zone and the refuse inlet, whereby the refuse,immediately before or within the gasification zone itself, is exposed toa further heating, resulting in the maximum extent of and a hightemperature in the gasification zone. Even if all the steam should beintroduced near the gasification zone, part of it will, however, makeits way towards the refuse inlet and provide for the supply of heat tothe refuse required for the degasifying.

To ensure the most uniform heating of the refuse over each retortcross-section, it has proved to be most expedient that a retort is usedhaving a substantially circular cross-section, whereas by the use ofretorts having an oval or polygonal cross-section rather uneventemperature distributions may arise, reducing the capacity of theretort.

Gradually as the refuse moves through the retort, its volume will bereduced as a result of the destruction, and to ensure that the whole ofthe refuse is influenced by steam, it may be expedient that a retort isused, the internal cross-sectional area of which diminishes in directionfrom the refuse inlet towards the ash outlet.

In case within a retort the superheated steam is brought directly toaffect the cold refuse just introduced, it may have the effect that theheating of the refuse will take place relatively slowly, in any case inthe part of the retort being next to the inlet, for which reason thispart will have to be particularly long, and further that the effect inthe longitudinal direction of the retort, reckoned from the inlet, therise in the temperature will in too high a degree be dependent on thewater content of the refuse. Therefore, it may be expedient that therefuse is preheated to a temperature of between 500 and 700C before itis influenced by the superheated steam, preferably before being fed intothe retort, so that it is ensured that there is no free water in therefuse, when the refuse gets into contact with the superheated steam,that the latter will not be condensed by the refuse, and that the refusewhen reaching the active part of the retort invariably will have aboutthe same temperature so that a fairly even temperature rise can beensured under all conditions in connection with the movement of therefuse through the retort.

The said preheating of the refuse may be carried through in a specialpreheating chamber of the retort, but most expediently it is performedin a special inlet to the retort. Provided this preheating takes placein an upwards open refuse feed shaft, the latter will work as av lead-insluice for the refuse since in the shaft the steam formed by thepreheating will provide a little below the free surface of the refuse,provided this does not sink below a certain level, a pocket of saturatedsteam, which on the top is limited by a thin layer of water, whichtogether with the steam pocket will have the effect of driving the gasescontained in the refuse out above the refuse, so that the refuse is fedinto the retort with the slightest possible content of air.

Even though by the new method as an essential part of the amount of heatrequired for the heating of the refuse has to be fed to the refuse inthe form of superheated steam, it may, however, be an advantage that theretort wall or a part thereof is simultaneously directly heated. Thisdirect heating, which in particular is of importance to the parts of theretort, which in direction with the inlet are at a distance from theretort zone with maximum temperature, has the effect that along the wallof the retort no circumferential zones are formed having a temperaturesubstantially lower than that obtained in the centre of the retort, and,at the same time, the said external heating ensures that the steamregains part of the heat previously given off to the refuse, which againhas the effect that the length of the retort may be rendered as short aspossible with consideration to the necessary decomposition times at thetemperatures obtained. The heating from outside may be obtained by meansof a heating medium flowing through ducts in or surrounding the retortwall. Provided further the heating medium is conducted in counterflowwith the refuse over a section between the refuse inlet and the part ofthe gasification zone next to the gas outlet, the external wall will getthe highest temperature there where the steam is to have the highesttemperature.

Most appropriate as heating medium there is used combustion productsfrom the steam generating device, so that no special heating apparatusis required.

Normally the refuse ought to be fed into the retort with a minimum ofatmospheric air contained therein, but in spite hereof, frequently itmay be advantageous simultaneously with the superheated steam tointroduce preferably preheated gasification means other than steam, suchas for example carbon dioxide, atmospheric air or enriched air. As arule it will only be a question of feeding atmosphere air, since theadding of other gasification means, which may mean an essentialadvantage, for economic reasons will depend upon whether there areactivities in the vicinity of the refuse destruction plant, whichproduce such gasification means as a kind of refuse products, whichwithout charge or against a small payment can be placed at the disposalof the refuse destruction plant. Adding atmospheric air along with thesteam will have the effect that the total amount of air introduced intothe retort can be kept under close control and adapted to requirements,which is not the case if large quantities of atmospheric air are allowedto be introduced along with the refuse itself. I

As appears from the above, for obtaining a satisfactory result it isnecessary that the superheated steam is able to contact the degassed,carbonaceous refuse. To

ensure this it may be expedient to use a retort having a rotatableretort wall, which continuously or discontinuously is brought to rotateduring the gasification of the refuse. Such a rotation of the retortwall will have the effect that, besides the movement of the refuse ma- 7terial through the retort in its longitudinal direction, transversemovements of the material are produced, whereby it is prevented thatobstructions, which are impervious to the steam, and ducts, which muchtoo easily may be penetrated by the steam, are formed, andsimultaneously ash formed is shaken away from the surface of the refuse,so that this may all the time participate in the gasification process,and the amount of free carbon in the ashes or the slags is therebyfurther reduced.

If a vertical retort having a rotatable retort wall is used, the pile ofrefuse material resting on the bottom of the retort or rather on thelayer of ashes and slag will have its outer portions bearing against thewalls of the retort withdrawn in the rotation of the retort wall, sothat the pile is exposed to a torsional stress causing the shaking ofthe material aimed at. In this case the strongest effect will usually beachieved, if the rotation of the retort wall is discontinuous, so thatthe formation of a slide layer between the refuse material and theretort wall is avoided, which otherwise might reduce the ability of theretort wall to withdraw the refuse material in its rotation.

Most expedient it is, however, is case a retort is used having arotatable retort wall, the axis of which is horizontal or extendsoblique downwards in the flow direction of the refuse, i.e. to use aretort, which on the whole corresponds to a rotary kiln. In such aretort it is possible to obtain a perfectly uniform heating of thematerial and a most effective removal of the ashes from the freesurfaces of material, so that a maximum destruction of the refuse isachieved.

In this case it is to be preferred that the feeding of the refuse intothe retort and the removal of ashes from the retort is adjusted in sucha manner that the outlet end of the retort is maintained completelyfilled with partly or completely destroyed refuse, so that there is nofree connection between the gasification zone and the gas outlet,whereby it is prevented that larger quantities of steam pass through theretort without substantially contributing to the heating of the refuseand the gasification, and that soot and tarry particles carried awaywith the gas and other substances formed during the degasifying andgasification processes, but not yet converted into gas, are retained bythe material filling the retort, so that these substances get time toreact with the vapour and other gasification means added, if any, andthe gas leaving the retort is free from impurities, which are difficultto remove.

The relatively large quantities of steam required for the heating of therefuse and being in substantial excess compared with the quantities ofsteam serving only as gasification means, leave the retort along withthe gas, wherefore, for heat economic reasons, it is still morenecessary than in the case of the known refuse gasification methods toensure that the gas, after having left the retort, preferably aftersolid impurities such as ashes particles have been removed from the gas,for instance in a cyclone, is conducted through heat exchangers and thenthrough a condenser for condensation of the major part of the vapourexisting in the gas, whereby a very great part of the heat used for thegeneration and superheating of the steam can be regained and utilizedfor preheating of refuse, gas combustion air for the fire of the steamboiler, air or other gasification means for the retort, and feed waterfor the steam boiler.

The condensed water may, if need be, be used as feed water for theboiler after removal of suspended ashes, dissolved salts and dissolvedgases.

However, for heat economic reasons, it may often be expedient torecirculate part of the hot vaporous gas to the retort.

After the vapours having been condensed by the gas, this may, forremoval of any in preceding plant parts not yet removed substances, suchas ashes, ammonia, hydrogen chloride, hydrogen sulphide, and othergases, be conducted to a venturi scrubber, where it is washed withwater.

From the venturi scrubber the gas can be conducted to a gas-holder or alike gas accumulator and from there to consumption, for instance to theboiler producing the steam. The water from the venturi scrubber willafter sedimenting of ashes contain for instance hydrogen chloride,hydrogen sulphide, ammonia, ammonium chloride, and ammonium sulphide inso poor a concentration that, from the point of view of contamination,it is harmless. Further, before discharge into the sewer, the free acidsmay even be neutralized by lime milk.

In this connection it should be mentioned that the gas volume from aplant to carry through the new method will be far smaller andaccordingly more economical and easy to cleanse effectively than theflue gas volume from an incinerator, in which the combustion air is indirect contact with the refuse, so that the flue gas has a large contentof nitrogen, which will load a cleansing plant for the flue gas.

Up to the priority date of the present patent, experiments on a grandscale with the new method have not been practicable without the dangerof open use, but the practicability of the method has been tested inmore primitive experimental plants, where, however, it was not possibleto carry out heat economy analyses.

The gasification under the presence of the very large quantities ofpreheated vapour has the greatest interest in regard to the practicalutilization of the new method, and, among others, this is based upon andsupported by laboratory gasification experiments, in which as refuserubbish from a mainly inhabitated town area was used, which rubbish wasdisintegrated and carefully mixed.

The gasification was undertaken in a rotating 1000 mm long Pythagoreantube, carried through another electrically heated 600 mm long stationaryPythagorean tube, and to which by means of a worm gear with adjustablenumbers of revolutions refuse was continuously fed, and through a steamtube, closed at the end but provided with side openings and extendinginto the rotating Pythagorean tube, steam was fed coming from alaboratory steam generator and superheated to about 400C. The bottom endof the rotating Pythagorean tube opened into an ashpit, from which thegas could be sucked up, and after condensation in a condenser cooled bymeans of cold water conducted to a gasmetre. The ashpit was so designedthat it was possible to introduce a Ni-CrNi thermo element housed in aquartz tube formeasuring the temperature over the greatest part of thelength of the rotating Pythagorean tube.

In the case of an experiment, chosen by way of an example, the rotatingPythagorean tube was heated to about 1050C, and immediately after theend of the steam tube the steam was thereby superheated to lO50C, whichtemperature, as a consequence of the steady supply of heat through thewalls of the rotating Pythagorean tube, was maintained up to about 300mm from the end of the rotating Pythagorean tube.

When the working conditions had been stabilized after some hours,analyses were made, which gave the following results:

Destroyed quantity of rubbish per hour: 0.5 kilo Produced gas liberatedfor vapour through condensing thereof: 0.4 normal cubic metreComposition of the gas in volume percentages:

H vO per cent.

CH 5 per cent.

CO 24 per cent.

CO 18 per cent.

C l-L, 3 per cent.

The upper calorific value of the gas (P1,): 3.200

kcal/normal cubic metres The lower calorific value of the gas (H 2.900

kcal/normal cubic metres Ashes: White with no measureable carboncontent.

What we claim is:

1. A method for disposal of refuse by degasification and gasificationthereof in a retort to convert the refuse into gases and ash products,the retort having an inlet for feeding the refuse into the retort and anoutlet for the ash products and for exhausting the gases from theretort, comprising the steps of continuously moving the refuse throughthe retort; continuously introducing superheated steam at a temperatureof at least 900C into the refuse in the area of the inlet to heat therefuse within the retort and in such amount that, within the retort, therefuse is degasified and is then gasified in a zone; and maintaining atemperature of at least 900C in said zone.

2. A method as set forth in claim 1 wherein the superheated steamintroduced in the retort has a temperature which exceeds 1200C.

3. A method as defined in claim 1, wherein the superheated steam isadditionally introduced into the retort between said zone and saidinlet.

4. A method as defined in claim 1 further comprising the step ofpreheating the refuse to a temperature of between 500C and 700C prior tothe step of heating the refuse by the superheated steam.

5. A method as defined in claim 4 wherein the step of preheating therefuse takes place prior to the refuse being fed into the retort throughthe inlet thereof.

6. The method as defined in claim 1, further comprising the step ofheating the retort along at least parts of the walls thereof toindirectly heat the refuse within the retort and supplement the heatingof the refuse being heated by the superheated steam.

7. The method as defined in claim 6 wherein the heating of the walls ofthe retort is accomplished by passing a heating medium in a counter flowwith respect to the refuse and over a section of the retort disposedbetween the inlet of the retort and a portion of the gasification zonenext to the outlet of the retort.

8. A method according to claim 6 further comprising the step ofproducing the superheated steam in a steam generating plant having asteam generator which is heated by flue gases from a fireplace utilizingthe gases produced by the gasification of the refuse and the flue gasesfrom the steam generating plant are used as the heating medium.

9. A method as defined in claim 1 further comprising the step of alsointroducing into the retort, simultaneously with the introduction of thesuperheated steam, additional gasification agents selected from a groupconsisting of carbon dioxide, atmospheric air and enriched air.

10. A method as defined in claim 1, further comprising the step ofrotating the walls of the retort about the longitudinal axis of' theretort during the gasification of the refuse. I

1 l. A method as defined in claim 10 further comprising the step offeeding the refuse into the retort at such a rate and removing the ashproducts from the retort at such a rate that the outlet end of theretort is maintained completely filled with partially or completelyconverted refuse.

l2. A method as defined in claim 1 wherein the heating of the refuse bythe superheated steam produces gaseous products which include fuel gasesand further comprising the step of recovering the gaseous products. l I!

2. A method as set forth in claim 1 wherein the superheated steamintroduced in the retort has a temperature which exceeds 1200*C.
 3. Amethod as defined in claim 1, wherein the superheated steam isadditionally introduced into the retort between said zone and saidinlet.
 4. A method as defined in claim 1 further comprising the step ofpreheating the refuse to a temperature of between 500*C and 700*C priorto the step of heating the refuse by the superheated steam.
 5. A methodas defined in claim 4 wherein the step of preheating the refuse takesplace prior to the refuse being fed into the retort through the inletthereof.
 6. The method as defined in claim 1, further comprising thestep of heating the retort along at least parts of the walls thereof toindirectly heat the refuse within the retort and supplement the heatingof the refuse being heated by the superheated steam.
 7. The method asdefined in claim 6 wherein the heating of the walls of the retort isaccomplished by passing a heating medium in a counter flow with respectto the refuse and over a section of the retort disposed between theinlet of the retort and a portion of the gasification zone next to theoutlet of the retort.
 8. A method according to claim 6 furthercomprising the step of producing the superheated steam in a steamgenerating plant having a steam generator which is heated by flue gasesfrom a fireplace utilizing the gases produced by the gasification of therefuse and the flue gases from the steam generating plant are used asthe heating medium.
 9. A method as defined in claim 1 further comprisingthe step of also introducing into the retort, simultaneously with theintroduction of the superheated steam, additional gasification agentsselected from a group consisting of carbon dioxide, atmospheric air andenriched air.
 10. A method as defined in claim 1, further comprising thestep of rotating the walls of the retort about the longitudinal axis ofthe retort during the gasification of the refuse.
 11. A method asdefined in claim 10 further comprising the step of feeding the refuseinto the retort at such a rate and removing the ash products from theretort at such a rate that the outlet end of the retort is maintainedcompletely filled with partially or completely converted refuse.
 12. Amethod as defined in claim 1 wherein the heating of the refuse by thesuperheated steam produces gaseous products which include fuel gases andfurther comprising the step of recovering the gaseous products.